1. Field of the Invention
The present invention relates to an electroluminescence display device, and more particularly, to an organic electroluminescence display device and a method of fabricating the same.
2. Discussion of the Related Art
A cathode ray tube has been widely used as a display device such as a television and a computer monitor. However, the cathode ray tube has large size, heavy weight, and high driving voltage. Therefore, flat panel displays having characteristics of being thin, light weight, and low in power consumption have been in demand. The flat panel displays include a liquid crystal display device, a plasma display panel device, a field emission display device, and an electroluminescence display device.
The electroluminescence display device may be categorized into an inorganic electroluminescence display device and an organic electroluminescence display device depending upon a source material for exciting carriers. The organic electroluminescence display device has drawn a considerable attention due to its high brightness, low driving voltage, and natural color images from the entire visible light range. Additionally, the organic electroluminescence display device has a great contrast ratio because of self-luminescence. The organic electroluminescence display device can easily display moving images due to its short response time of several microseconds, and is not limited by a viewing angle. The organic electroluminescence display device is stable at a low temperature, and its driving circuit can be easily fabricated because it is driven by a low voltage. Besides, a manufacturing process of the organic electroluminescence display device is relatively simple.
In general, an organic electroluminescence display device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Since its luminous mechanism is similar to a light emitting diode, the organic electroluminescence display device may be called an organic light emitting diode (OLED).
FIG. 1 is a schematic cross-sectional view illustrating a related art organic electroluminescent display device. As shown in the related art organic electroluminescent display device, an anode electrode 50 is disposed on a glass substrate 12. A hole injection layer 52, a luminous layer 54 and an electron injection layer 56 are formed in series on the anode electrode 50. A cathode electrode 58 is formed on the electron injection layer 56. Namely, the hole injection layer 52, the luminous layer 54 and the electron injection layer 56 are disposed between the anode electrode 50 and the cathode electrode 58. When the driving voltage is applied to the anode and cathode electrodes 50 and 58, the holes of the hole injection layer 52 and the electrons of the electron injection layer 56 are injected into the luminous layer 54. The injected electrons are combined with the injected holes, thereby producing the electron-hole pairs (the excitons). The electron-hole pair has a lower energy than when it is separated into the electron and the hole. Therefore, an energy gap occurs between the combination and the separation of electron-hole pairs, and this energy is converted into light by the luminous layer 54. That is, the luminous layer absorbs the energy generated due to the recombination of electrons and holes when a current flows.
An organic material for the organic electroluminescent display device is classified into an organic polymer and an organic monomer. Thus, the organic electroluminescent display device is also classified into a polymeric organic electroluminescent display device, a monomeric organic electroluminescent display device, and a mixture thereof. Further, the organic electroluminescent display device can be a combined organic electroluminescent display device that uses both the organic polymer and the organic monomer.
The monomeric organic electroluminescent display device is generally fabricated through a thermal evaporation method that deposits a low molecular substance. Such thermal evaporation method forms unit pixel pattern using a shadow mask. That is, the low molecular substance is thermal-evaporated onto the substrate by passing though pre-patterned openings of the shadow mask. However, the unit pixel becomes smaller in size because the deposition is repeated using the shadow mask to form R/G/B sub-pixels. The shadow mask is not utilized for fabricating the large display device because the shadow mask sags under its own weight and size.
Meanwhile, a high molecular substance is deposited on the substrate using a spin coating method. Since the high molecular substance has a superior impact resistance and makes qualitative and strong thin films as compared to the low molecular substance, it has been used for the organic light emitting diodes. However, when the full-color type organic electroluminescent display device is fabricated using the high molecular substance, the spin coating method is avoided because it produces some disadvantages, such as a high costs of production and a thickness deviation between the central part and the peripheral part of the substrate. Namely, the spin coating method produces a thin film having nonuniform thickness. Therefore, an ink-jet technique is utilized as a new method of forming an organic electroluminescent display device.
FIG. 2 is a schematic cross-sectional view of an organic electroluminescent display device fabricated by an ink-jet technique according to a related art.
In the case of ink jet process, the high molecular substance is simultaneously deposited and patterned, and thus the fabrication process can be conducted relatively in a short time. As shown in FIG. 2, a plurality of first electrodes 50 are formed in pixel regions on a transparent substrate 12. A separator 60 is also formed on the transparent substrate 12 among the first electrodes 50 and overlaps the side portions of the first electrodes 50. Thereafter, hole injection/transporting layers 52 and luminous layers 54 are sequentially formed on the first electrodes 50 by emitting the organic polymer from an ink-jet toner. At this time, each hole injection/transporting layer and each luminous layer 54 are disposed in intervals defined by the separators 60. Further, a second electrode 58 is formed over the whole of the transparent substrate 12, thereby completing the organic electroluminescent display device. However, the above-mentioned process is rather complicating and takes relatively long time because the high molecular substance is respectively deposited in the pixel region.
FIG. 3 is a schematic plan view especially illustrating separators and red (R), green (G) and blue (B) pixels of an organic electroluminescent display device that are fabricated by an ink-jet technique according to a related art. As shown FIGS. 2 and 3, the separator 60 divides the substrate into the lattice-like red (R), green (G) and blue (B) pixels. Since the separator 60 protrudes from the substrate 12 and then makes the step, as shown in FIG. 2, the second electrode 58 is broken and cut when forming the second electrode 58 over the transparent substrate 12.